Process for replenishing nickel plating electrolyte



ETAL

Feb. 13, 1951 E. .1. RoEHL' PROCESS FOR REPLENISHING 'ICKEL PLATINGELECTROLYTE Filed April 22,

INVENTORS Ibn/Awa 170mm/ Foe/a wafrfwhrszy Patented Feb. 13, 1951PROCESS FOR REPLENISHING NICKEL PLATIN G ELECTROLYTE Edward JudsonRoehl, Little Silver, and Andrew Wesley, Plainfield, N. J., assignors toThe International Nickel Company, Inc., New York, N. Y., a corporationof Delaware l Application April 22, 1948, Serial No.22,727

11 Claims. l

The present invention relates to a process for electrodepositing nickelemploying an insoluble anode wherein the nickel content and acidity ofthe electrolyte employed are maintained substantially contant and, moreparticularly, to a feasible process for nickel plating with an insolubleanode wherein the nickel content and acidity of the nickel platingelectrolyte are maintained substantially constant by electrolytic meansand the electrolyte is maintained -in condition to permit the productionof sound nickel electrodeposits therefrom.

It has been recognized in the art that insoluble anode processes forelectroplating nickel offer distinct advantages in many fields. Forexample, in the plating of steel strip and wire, the electroforming offine screen material and parts of intricate or irregular shape, platingthe inside of tubes, etc., it is important that the distance separatinganode from cathode be kept substantially constant and this is mostreadily accomplished by employing anodes which are substantiallyinsoluble in the plating bath. As recognized by those skilled in theart, the metal ions for plating must be provided by the plating bath assubstantially no metal ions are supplied by the anode in the insolubleanode plating process. This results in the disadvantage that the bat-hused for plating with an insoluble anodel becomes depleted in metal ioncontent and, since the metal ions plated out are replaced by anequivalent amount of hydrogen ions by electromechanical reactions, theacidity of the bath increases. Due to the combination of these factors,the bath eventually becomes unsuitable for further plating unless stepsare taken to replenish the metal ion content and to decrease theacidity.

In `the plat-ing of copper using insoluble anodes, the copper platingsolution can be re' plenished simply by passing it over copper scrap,etc., as the solution reacts with the copper to increase the coppercontent and decrease the acidity of the solution. However, nickelpresents an entirely different problem and, when replenishment of adepleted nickel electrolyte is attempted in a similar manner, theresults are unsatisfactory and the method impracticable. For example,the rate of corrosion of the nickel scrap, etc., in the electrolyte isvery low and, in addition, nickel has a` strong tendency towardpassivity in the electrolyte, particularly the chloride-freeelectrolyte. These and other factors require that the volume of solutionand surface area of scrap used in the replenishing system be soimpracticably large that no satisfactory method employing this principlehasyet been devised. Other` methods have been tried or proposed for thereplenishment of depleted f are high and it is diilicult to maintain thediaphragm in good operating condition. Although many attempts have beenmade to remedy these and other shortcomings, none, as far as we areaware, was entirely successful when carried into commercial practice.

Thus, a definite need exists in the art to pro vide a successful methodwhereby nickel electrodeposits can be produced employing an insolubleanode, and at the same time, the nickel ion content and acidity of theelectrolyte are maintained substantially constant in ranges where theproduction of sound nickel deposits at a satisfactory rate is feasible.

It is an object of the present invention to provide a process forelectrodepositing nickel from a cell having an insoluble anode whereinthe nickel content and pH of the electrolyte are maintainedsubstantially constant.

It is still another object of the present invention to provide a methodfor replenishing the nickel content and maintaining the acidity of theelectrolyte used in electrodepositing nickel employing an insolubleanode.

It is a further object of the present invention to provide anelectrochemical method for simultaneously providing a supply of nickelions to and diminishing the acidity of a nickel plating electrolyte.

It is a still further object of the present invention to provide amethod for emciently dissolving nickel electrolytically. It is anotherobject of the present invention to provide a method whereby electrolyticnickel may be dissolved electrolytically at high edlciency in achloride-free nickel plating bath and, at the same time, to maintainthebath in condition that satisfactory nickel electrodeposits may beproduced therefrom at feasible rates of deposition and over long periodsof time.

Other objects and advantages of the presen invention will becomeapparent from the following description taken in conjunction with thedrawing which depicts schematically an embodiment of the invention.

Broadly stated, the process of the invention comprises electrodepositingnickel at a plating cathode by passing a direct current through an acidaqueous nickel sulfate plating electrolyte from an insoluble anode tothe plating cathode (said insoluble anode and said cathode being atleast in part immersed in said electrolyte), replenishing the nickelcontent of depleted plating electrolyte by passing a direct currenttherethrough between electrodes having a nickel surface exposed to theelectrolyte at a current density of about l to 25 amperes per squarefoot of effective anode surface exposed to the electrolyte, reversingthe direction of said direct current at intervals of about 1 to about 50seconds, and'v electrodepositing nickel from the thus-replenishedelectrolyte. The reversal of direct current flow during the replenishingoperation will usually be repeated as often as required to replacenickel removed from tlie electrolyte in prior electrodeposition and toprovide the nickel concentration required for further electrodepositionof nickel from the replenished electrolyte. In practice, the electrolytecan be transferred from the electroplating unit to the replenishing unitand returned either at a continuous or intermittent rate of transfer. I

It is critical that three separate operating factors-be met in carryingoutthe present invention. That is, in the replenishing operation thenickel `surface of the anode "must be maintained in an active conditionand there must be an economically feasible difference between anodeeiciency and a. lower cathode eiciency, while in the plating operationan opposing high electroplating efficiency, e. g., cathode efciency,must be maintained. Inability to meet any one of these operating factorswould vitiate the entire process, and the present invention resides inthe unexpected discovery of a set of conditions which enables satisfyingthese critical factors simultaneously.

These factors are met simultaneously by the cona nickel sulfate platingelectrolyte such as described herein. Y

In practice, the aqueous acid nickel sulfate electrolyte employed in theprocess contains or is comprised essentially of about 100 to aboutv 400grams of nickel sulfate per liter of electrolyte,

buffered with labout to about 50 grams of boric acid per liter, saidelectrolyte having a pH of about 0.75 to about 3.5, a temperature ofabout 100 to about 160 F., ,fand being substantially devoid ofchlorides, i. e., containing not more than about 1 gram per liter ofchloride ion, preferably not more than about 0.01 to 0.1 gram per liter.Preferably, the electrolyte contains about 250 to 350 grams per liter ofnickel sulfate, about to 40 grams per liter boric acid, and has a pH ofabout 1 to 2. It is to be pointed out that the object of thereplenishing operation is to maintain the composition of the electrolyteduring the electroplating operation within the aforesaid ranges in orderthat plating may be satisfactorily carried out for long periods of time.The current density employed in the electroplating operation may be fromabout 25 to about 2000 amperes per square foot. By way of example, itmay be said that an aqueous electrolyte containing or comprisedessentially of about 300 grams per liter of nickel sulfate and about 30grams per liter of boric acid, operated at a pH of about 1.5 and atemperature of about F., has been found to be satisfactory and to giveoptimum overall eniciency for the system in a closed circuit oflseparate electroplating and replenishing operations.

Control of electrolyte pH is also a critical aspect of the process as awhole because it has been found that when the pH is lower than about0.75 the efficiency of electroplating is impracticably low, while whenthe pH is higher than about 3.5 the net gain'of nickel dissolved in thereplenishing operation is impracticably low.

The nickel electrodes employed in the replenishing operation arepreferably made of electrolytic nickel, although the nickel electrodescan be any, anode material which will supply dissolved nickel to theelectrolyte when it acts as anode during the replenishing operation, e.g., cast or Wrought nickel anodes, and which does not introduce harmfulamounts of contaminants into the nickel plating electrolyte.

A feature of the present invention is that a satisfactory regenerativeplating system for nickel plating with an insoluble anode is provided inwhich an electrolytic replenishing operation has been devised whereinnickel is dissolved anodically at a favorably higher rate than it isdeposited cathodically, i. e., wherein the anode eiiciency is favorablyhigher than the cathode eciency. When this condition is achieved, the

.function of the replenishing operation to supply nickelions and todecrease the acidity in-the platingl electrolyte which has been depletedin nickel ions and has become more acid in the plating operation isrealized. In the present invention, this has been provided by control ofcritical features affecting the replenishing operation and including thereversal of the flow of the direct current applied between theelectrodes employed in the replenishing operation, the time cycle ofcurrent reversal, the current density effective upon the electrodes, thepH and composition of the electrolyte. etc. It has been found thatreversal ofthe direct current is essential and that the direct currentmust be reversed at intervals of from about 1 to about 50 seconds(cycles of about 2 to about 100 seconds), preferably at intervals ofabout 3 to 7 seconds e. g., 5 seconds. It is important to avoidreversing intervals shorter than about l second (for example, asrepresented by 60-cycle alternating current) or longer than about 50seconds, as it has been found that both conditions produce lowelectrochemical e'iciency in the replenishing operation with the resultthat the net gain of nickel dissolved becomes too low for successfuloperation.' In addition to the foregoing, it is critical that the netcurrent density applied on each electrode in each direction of currentnow be maintained within the aforementioned range of about 1 .to about25 amperes per square foot in the replenishing operation, preferablyabout 5 to l5 amperes per square foot, e. g., 10 amperes per squarefoot, because at current density values below the minimum the rate ofdissolution of nickel becomes impracticably low, while at currentdensity values above the maximum the rate of deposition of nickel in thereplenishing operation approaches and may even exceed the rate ofdissolution of nickel therein, with the result that the net gain ofnickel dissolved in the electrolyte decreases below practicable limits.It is preferred that 'the time intervals and current density in thereplenishing operation be so chosen that the same number of ampere hoursper square foot is alternately applied on the cathode side in eachdirection of current flow, e. g., the same current density and the sametime interval for each reversal of current flow, as otherwise one set ofelectrodes would dissolve faster than the other set or might even growat the expense of the other. In addition, some difficulties might beencountered by way of growths on one set of electrodes which mightproduce short circuits, particularly when the electrodes are closelyspaced. From a practical standpoint, it is desirable to replace bothsets of electrodes at the same time and to employ electrodes ofsubstantially the same area.

As indicated hereinbefore, it is important that conditions in thereplenishing operation be such that the rate of solution of nickel atthe anode is favorably higher than the rate of deposition of nickel atthe cathode, i. e., the anodic eficiency of the operation must be higherthan the cathodic efiiciency. Thus, anode efficiencies should be atleast as high as about 50% and may be as high as about 100% whilecathode efliciencies may be as low as about 25% or 30%, but should notbe over about 70%. The difference between anode and cathode eiiicienciesmay be about to about 70% and preferably should be about 50% or more.

The process of the present invention is preferably carried out using asubstantially chloridefree nickel plating bath, i. e., a bath containingless than 0.1 gram of chloride ion per liter. However, amounts ofchloride up to about 1 gram per liter, e. g., about 0.01 to 1 gram perliter, do not adversely aiect operation of the process. It has beenfound that if the chloride ion content substantially exceeds thisamount, the eiiiciency of the replenishing operation with respect to netgain of nickel dissolved is markedly reduced and other difficulties maybe encountered.

It has been found that the electrochemical efticiency 'of thereplenishing operation is highest when both the current density and thepH of the electrolyte are low. If either of these factors approaches thepermissible maximum recited herein it is preferred that the other factorbe maintained near the minimum recited herein. For example, whenreplenishing is carried out at a current density of 25 amperes persquare foot it is preferred that the pH of the electrolyte be about 1 orless. If the current density is only 1 ampere per square foot, then thepH of the electrolyte can be fas high as about 3.5. Various means may beemployed to insure meeting the various requirements set forthhercinbefore. For example, in a case where the current eiciency withrespect to net gain of dissolved nickel in the replenishing operation isthe same as the current emciency with respect to deposition of nickel inthe plating operation and the replenishing operation is conducted with acurrent density of l0 amperes per square foot and the plating operationis operated at 50 amperes per square foot, the electrode area for eachset of electrodes in the replenishing operation should be at least aboutve times as great as the cathode area in the plating operation if it isdesired to maintain substantially equal rates of depletion andreplenishment of nickel in the system. By observing any-changes in thenickel ion content and/or pH of the electrolyte, it is possible tomaintain control of the overall performance of the system. For example,a drop in nickel content or increase in acidity indicates that nickel isbeing depleted more rapidly in the plating operation than it is beingdissolved in the replenishing operation, while an increase in nickelcontent and decrease in acidity indicates the opposite. Control measuressuch as increasing the electrode area in the replenishing operation canbe employed to maintain the nickel content and acidity of theelectrolyte substantially constant when the process embodying theinvention is in operation. Other control measures may also be employed.For example, the rate of nickel dissolution in the replenishingoperation can be increased or decreased by respective increases ordecreases in current density without changing conditions in the platingoperation. Likewise, the current density in the plating operation can becontrolled to increase or decrease the rate of nickel depletion in theelectrolyte, etc,

It is to be noted that small amounts of impurities which do notinterfere with the working of the plating operation can be tolerated inthe nickel sulfate electrolyte. In order to conform with acceptedstandards for good nickel plating practice, the impurities in theelectrolyte can be tolerated within the following ranges:

Grams per liter If calcium is present, it may be included in amountsfrom about 0.002 gram per liter up to the concentration at which CaSO4precipitates.

Suitable equipment for carrying out the invention comprises anelectroplating unit comprising an insoluble anode and a cathode, andhaving a direct current supply'; a replenishing unit comprising one ormore nickel electrodes of each polarity andprovided with a directcurrent supply having current reversing means; means for circulatingelectrolyte between the two units (e. g., pumps, strrers, etc.) andconventional auxiliary equipment such as heating means, etc., forcontrolling conditions, etc., in the equipment. The electroplating unitand the replenishing unit may be contained in separate cells (or tanks)or in separate compartments of the same cell (or tank), or may even becontained in the same compartment.

In order that those skilled in the art may have a better understandingof the present invention, the following illustrative example is givendescribing the operation of a regenerative plating system for platingnickel using an insoluble anode.

EXAMPLE A recirculating plating and replenishing system illustratedschematically in the drawing was set up comprising a plating tank l anda replenishing tank 2 of approximately equal size connected stirringdevices in each tank and heating means y. 1 to maintain the electrolytetemperature.

The platlng' tank was provided with a cathode 8 and an insoluble anode 9and the replenishing tank was provided with a row of live equalelectrolytic nickel electrodes I disposed in face-to-face relationshipand having alternate electric polarity. In this manner, four sets ofelectrode surfaces of equal effective area and of opposite polarity wereprovided. Each tank was provided with an independent direct currentpower supply Il and I2, that to the replenishing tank being providedwith current reversing means I3. A chloridefree aqueous nickel sulfateplating bath was established containing about 300 grams of nickelsulfate per liter of electrolyte (72.5 grams per liter of nickel) and-30grams of boric acid per liter. The bath was pllled according to standardprocedure. Thereafter, the bath pH was adjusted to 1.3 and thetemperature was adjusted to and maintained at about 130 F. for theduration of the run, and the electrolyte was circulated between thetanks at a rate equivalent to a complete circulation of the electrolyteevery 2 hours. A run of 28 hours duration was then made during which thecurrent density in the plating cell was maintained at about 40 amperesper square foot and that in the replenishing cell was maintained atabout 10 amperes per square foot. The direct current flow in thereplenishing cell was reversed at 5-second intervals. The effective areaof one set of electrodes (effective area of each polarity) in thereplenishing cell was about 8.67 times the cathode area in the platingcell. At the end of the run, the electrolyte pH was 1.2 and the nickelcontent was '70.7 grams per liter. The metal deposit obtained during therun was stripped from the cathode and machined into tensile test stripspecimens. Results of tests on these specimens are shown in Table I,together with comparative results obtained on nickel electrodepositedfrom a standard Watts-type bath using soluble anodes.

TABLE I Test results 'on electrolytic nickel from sulfate bath.

Thickness, Hardness, Tensile Percent Elongaton in* in. VHN 1 Strengthlll 1%!! 2l .033 167 76,000 as 20 1s .osa 162 18,000 29 22 19 .030 o71.000 s0 22 19 1.034 14s v68,250 24 20 n. d.

n. d.=not determined.

l Vickers hardness number-l0 k load.

1 Test results on deposit obtaine from standard .Watts-type bathoperated at 170 F., pH 1.0, a cathode current density of 50 amperes persquare foot, containing 325 grams por liter nickel sulfate, 45 grams perliter nickel chloride and30 grams per liter of boric acid.

electroformed articles and the like. Operation of the present process isattended by numerous practical advantages. Thus, no anode bags areneeded in the plating operation; undesirable contaminants can readily beexcluded or eliminated from the electrolyte; close control of the anodeto cathode distance in the plating operation can be maintained;electrodes may be closely spaced in the replenishing operation; nickelis supplied to the electrolyte from nickel electrodes which can ,be'ofhigh purity, e. g., electrolytic nickel, and

',ode plating process, comparing favoroably even with soluble anodeplating processes.

It is to be pointed out that insoluble anodes made of lead preferablyshould not be used during the electroplating operation wherever slightlead contamination would .be harmful to the mechanical and/or physicalproperties of the resulting electrodeposited article. For example,although articles made in the plating operation using lead anodes havesatisfactory strength and ductility in the as-plated condition, thesearticles tend-to become weak and brittle upon annealing. More preferredinsoluble anodes may be fashioned from platinum or from platinum-basealloys containing other platinum-group metals. Thus, platinum alloyscontaining iridium up to 30% or rhodium up to 40% or ruthenium up to 15%may be used. As previously indicated, pure platinum may also be used,but if other platinumgroup metals are present they will be included inamounts exceeding about 0.01% each. Not only may platinum-base alloyscontaining palladium be employed but palladium-base alloys containing yplatinum will also give acceptable results. Other precious metals, suchas gold, also give acceptable results. Base metals havingcorrosion-resistant properties similar to tungsten may also be employedas well as tungsten. Among the nonmetallic materials, magnetite may bementioned as a suitable insoluble anode material. Graphite anodesproduce acceptable nickel electrodeposits, but this material is notpreferred because of its tendency to gradually disintegrate when used asanode in the electrolyte. The term insoluble anode is used herein in itsconventional sense and means a conducting material which is eithercompletely insoluble or which corrodes in the bath at such a low ratethat its replacement is yeconomical and practical and that it does notbuild up harmful soluble or insoluble impurities in the bath at anexcessive rate.

It will be recognized from the foregoing that the electrolyte should besubjected alternately to electroplating and replenishing operations.This will normally require circulating the electrolyte between theelectroplating and replenishing uriits at a rate suiiicient to maintainthe nickel concentration in the replenished electrolyte at a levelsuitable for further electroplating in the electroplating unit. Theproper rate of electrolyte transfer between these units necessary toachieve this result will vary with each application .since this ratedepnds upon many variable factors such as the volume of the platingelectrolyte, rate of extraction of nickel from the bath in the platingoperation, rate of replenishment of nickel in the replenishing opferation, etc. As indicated hereinbefore, these factors are controlled bythe current densities employed in both the electroplating andreplenishing operations, the number of current reversals during thereplenishing operation and the intervals at which the current isreversed in the replenishing operation and the pH and composition of theplating bath.

As indicated hereinbefore, any anode materials which will supplydissolved nickel to the electrolyte when acting as anode therein andwhich do not introduce harmful amounts of impurities such as iron, zinc,etc., into the electrolyte may be employed as electrodes in thereplenishing operation, but it is preferred that electrolytic nickelanodes be used. As those skilled in the art know, electrolytic nickel isa high purity nickel containing more than 99% nickel (including a smallamount of cobalt which is usually less than 1%).

Other satisfactory soluble anodes (i. e., anodes which show goodactivity and which dissolve readily in the nickel electrolyte)v whichmay be employed in the replenishing operation are cast and wroughtnickel anodes usually having analyses within the following ranges:

Although the present invention has been described in conjunction withpreferred embodiments. it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such variations and modifications are to be consideredwithin the purview and scope of the invention and the appended claims.

We claim:

l. The process for electrodepositing nickel which comprises establishingan aqueous nickel plating electrolyte essentially comprised of about 100to 400 grams per liter of nickel sulfate, about to 50 grams per liter ofboric acid, substantially devoid of chloride ions,` having a pH of about0.75 .to 3.5 and a temperature of about 100 to 160 F., electrodepositingnickel from said electrolyte in an electroplating unit having a sourceof direct current, at least one insoluble anode and at least one cathodeby passing a plating current from said direct current source throughsaid electrolyte from said insoluble anode to said cathode toelectrodeposit nickel at said cathode, ilowing said electrolyte to areplenishing unit comprising a source of reversible direct current inelectrical connection with a plurality of nickel electrodes,substantially replenishing the nickel content of said platingelectrolyte by passing therethrough between said electrodes directcurrent from said source of reversible direct current at a currentdensity of about l to 25 amperes per square foot while repeatedlyreversing the direction of said direct current at time intervals ofabout l to 50 seconds to obtain a net gain of nickel dissolved from saidelectrodes, flowing the thus-replenished electrolyte to anelectroplating unit and electrodepositing nickel from thethus-replenished electrolyte.

2. The process according to claim i wherein the aqueous nickel platingelectrolyte is essentially comprised of about 250 to 350 grams of nickelsulfate per liter of electrolyte, about 20 to 40 grams per liter ofboric acid, and has a pH of about l to 2, wherein the direct current inthe replenishing unit ilows at a current density of about 5 to 15amperes per square foot and wherein the direction of said current` isreversed at time intervals of about '3 to 7 seconds.

3. The process according to claim 2 wherein the current in thereplenishing unit ilows at a current density of about 10 amperes persquare foot and wherein the direction of said current is reversed atintervals of about 5 seconds.

4. The process according to claim 3 wherein the aqueous nickel platingelectrolyte is essentially comprised of about 300 grams per liter ofnickel sulfate, about 30 grams per liter of boric acid, and has a pH ofabout 1.5.

5. The process which comprises electrodepositing nickel by passing anelectrodepositing current from an insoluble anode immersed in an aqueousvnickel sulfate plating electrolyte substantially devoid of chloride ionand having a pH of about 0.75 to 3.5 to a cathodeW likewise immersed insaid electrolyte to electrodeposit nickel at said cathode, flowing saidelectrolyte t0 a replenishing unit. replenishing the nickel content ofsaid plating electrolyte insaid re\ plenishing unit by owingtherethrough between nickel electrodes direct current at a currentdensity of about 1 to 25 amperes per square foot from a source ofreversible direct current while reversing the direction of said directcfirrent at intervals of about 1 to 50 seconds. iiowing thethus-replenished electrolyte to an electrodepositing unit comprising aninsoluble anode and a cathode and electrocienositing nickel from thethus-replenished electrolyte.

6. The process for electrodepositing nickel which comprises establishingan aqueous nickel plating electrolyte essentially comprised of about 100to 400 grams per liter of nickel sulfate, about 10 to 50 grams per.liter of boric acid, substantially devoid of chloride ions, having a pHof about 0.75 to 3.5 and a temperature of about 100 to 160 F.,electrodepositing nickel irom said electrolyte in an electroplating unithaving a source of direct current, at least one insoluble anode and atleast one cathode by passing a plating current from said direct currentsource through said electrolyte from said insoluble anode to saidcathode to electrodeposit nickel at said cathode, flowing saidelectrolyte to a replenishing unit comprising a source of reversibledirect current in electrical connection with a plurality of electrolyticnickel electrodes, substantially replenishing the nickel content of saidplating electrolyte by passing therethrough between said electrodesdirect current from said source of reversible direct current at acurrent density of about 1 to 25 amperes per square footmwhilerepeatedly reversing the direction of said direct current at timeintervals of' about 1 to 50 seconds to obtain a net gain of nickeldissolved from said electrodes, flowing the thus-replenished electrolyteto an electroplatingl unit and electrodepositing nickel from thethus-replenished electrolyte.

7. Theprocess according to claim 6 wherein the aqueous nickel platingelectrolyte is essentially comprised of about 250 to 350 grams of nickelsulfate per liter, about 20 to 40 grams of boric acid per liter, and hasa pH of about 1 to 2, and wherein the direct current in the rel1plenishing unit ows at a current density o! about 5 to 15 amperes persquare foot and the in saidvelectrolyte a net gain of nickel Ydissolveddirection of said current is reversed at intervals o1' about 3 to 7seconds.

8. The process according to' lclaim 7 whereinthe current in thereplenishing unit iiows at' a 10. The process for electrodepositingnickel which comprises alternately subjecting to an electroplatingoperation and to a replenishing operation an aqueous nickel platingelectrolyte containing about 100 to 400 grams of nickel sulfate perliter of electrolyte.` about to 50 grams per liter of boric acid, havinga pH of about 0.75 to 3.5 and being substantially devoid of chlorideions, said electroplating operation comprising passing a plating currentthrough said electrolyte between an insoluble anode and a cathodeimmersed therein to electrodeposit nickel at said cathode, saidreplenishing operation being conducted in a replenishing unit having aplurality of electrolytic nickel electrodes in electrical connectionwith a source of reversible direct current and comprising passingthrough said electrolyte between said electrodes Vdirect current fromsaid source of reversible direct current at a current density of about 1to 25 amperes per square foot while reversing the direction of iiow ofsaid direct current at least once at intervals of about 1 to 50 secondsto' obtain from said electrodes.

11. The process according to claim 10 wherein the aqueous nickel platingelectrolyte is essen# tially comprised of about 4v250 to 350 grains ofnickel sulfate per liter, about 20 to 40 grams of boric acid per liter,and has a pH of about 1 to 2.

EDWARD JUDSON ROEI-IL. ANDREW WESLEY.

nErEltE-Ncns CITED The following references are ofrecord inthe le ofthis patent:

UNITEDl STATES PATENTS Number Name Date 1,003,092 Dow et al Sept. 12,1911 1,144,680 Allers June 29, 1915 1,885,148 Smith Nov. 1, 19322,431,949 Martz Dec. 2, 1947 2,449,422 Smith Sept. 14, 1948 2,449,495Lum Sept. 14, 1948 2,451,341 Jernstedt Oct. 12, 1948 2,470,775 Jernstedtet al May 24, 1949 FOREIGN PATENTS Number Country Date 310,099 GreatBritain Apr. 25, 1929 y 49,384 f Denmark Oct. 15, 1934 OTHER. REFERENCESTransactions of the Electrochemical Society, Watts, vol. 59 (1931), page381.

Blum et al., Transactions of the Faraday Society, vol. 31 (1935), page1208.

Finch et al., Transactions of the Faraday Society, vol. 33 (1937) page566.

1. THE PROCESS FOR ELECTRODEPOSITING NICKEL WHICH COMPRISES ESTABLISHING AN AQUEOUS NICKEL PLATING ELECTROLYTE ESSENTIALLY COMPRISED OF ABOUT 100 TO 400 GRAMS PER LITER OF NICKEL SULFATE, ABOUT 10 TO 50 GRAMS PER LITER OF BORIC ACID, SUBSTANTIALLY DEVOID OF CHLORIDE IONS, HAVING A PH OF ABOUT 0.75 TO 3.5 AND A TEMPERATURE OF ABOUT 100* TO 160* F., ELECTRODEPOSITING NICKEL FROM SAID ELECTROLYTE IN AN ELECTROPLATING UNIT HAVING A SOURCE OF DIRECT CURRENT, AT LEAST ONE INSOLUBLE ANODE AND AT LEAST ONE CATHODE BY PASSING A PLATING CURRENT FROM SAID DIRECT CURRENT SOURCE THROUGH SAID ELECTROLYTE FROM SAID INSOLUBLE ANODE TO SAID CATHODE TO ELECTRODEPOSIT NICKEL AT SAID CATHODE, FLOWING SAID ELECTROLYTE TO A REPLENISHING UNIT COMPRISING A SOURCE OF REVERSIBLE DIRECT CURRENT IN ELECTRICAL CONNECTION WITH A PLURALITY OF NICKEL ELECTRODES, SUBSTANTIALLY REPLENISHING THE NICKEL CONTENT OF SAID PLATING ELECTROLYTE BY PASSING THERETHROUGH BETWEEN SAID ELECTRODES DIRECT CURRENT FROM SAID SOURCE OF REVERSIBLE DIRECT CURRENT AT A CURRENT DENSITY OF ABOUT 1 TO 25 AMPERES PER SQUARE FOOT WHILE REPEATEDLY REVERSING THE DIRECTION OF SAID DIRECT CURRENT AT TIME INTERVALS OF ABOUT 1 TO 50 SECONDS TO OBTAIN A NET GAIN OF NICKEL DISSOLVED FROM SAID ELECTRODES, FLOWING THE THUS-REPLENISHED ELECTROLYTE TO AN ELECTROPLATING UNIT AND ELECTRODEPOSITING NICKEL FROM THE THUS-REPLENISHED ELECTROLYTE. 